In most computers having virtual memory management, the available address space for the computer's operating system and application processes exceeds the amount of physical RAM available. As shown in FIG. 1, a computer system 100 will typically have a central processing unit (CPU) 102, such as a Motorola 68040 or Intel 80486, a primary memory 104 (typically random access memory (RAM)), a secondary memory 106 (typically one more magnetic or optical disks), and a cache memory 108 for storing a copy of the memory locations most recently addressed by the CPU 102. The cache memory 108, or a portion of the cache memory, is often fabricated as part of the same integrated circuit as the CPU 102.
As indicated in FIG. 1, the CPU 102 is typically the fastest of the components in the computer system, followed by the cache memory 108, the primary memory 104 and finally the secondary memory 106. Also shown in FIG. 1 is a virtual memory unit (VMU) 110, which is often fabricated as part of the same integrated circuit as the CPU 102.